This invention relates to a method and ultrasound apparatus for detecting liquid flow velocity, and more particularly to determining two components of velocity and the true flow vector using a steered beam scanner and a transducer array cross-beam technique.
Doppler systems which transcutaneously insonify the bloodstream with a single ultrasonic beam can provide accurate blood velocity measurements only if the direction of the velocity vector coincides with the direction of the beam; if these directions do not coincide it is necessary to know the angle .theta. between the velocity vector and the beam. Various methods have been suggested for deriving two components of velocity from which the true velocity vector is readily calculated, but many such methods require plural insonifying beams. The system described by P. A. Perronneau et al, Proceedings of the 2nd World Congress of Ultrasonics in Medicine, Rotterdam, The Netherlands, June 1973, pp. 259-266, utilizes two crystals angled toward one another and symmetrically located at one extremity of a B-scan probe diameter.
Blood velocity is detected by measuring the Doppler shifts in frequency imparted to ultrasound by reflection from moving red blood cells. Accuracy in detecting the Doppler shift at a particular point in the bloodstream depends on defining a small sample volume at the required location and then processing the echoes to extract the Doppler shifted frequencies. In order to be able to place an appropriately small sample volume at any desired location in the bloodstream and examine velocity in the major vessels, in the vicinity of heart valves or inside the heart chambers, a Doppler modality is incorporated in a real time sector scanning imaging system. The system provides electronic steering and focusing of a single acoustic beam and enables the user to illuminate a small sample anywhere in the field of view of the instrument, whose location can be visually identified on a two-dimensional B-scan image. A Fourier transform processor faithfully computes the Doppler spectrum backscattered from the sampled volume, and by averaging the spectral components the mean frequency shift can be obtained. Further explanation is given in copending, commonly assigned application Ser. No. 936,111, filed on Aug. 23, 1978, E. Papadofrangakis and W. E. Engeler, "Directional Detection of Blood Velocities in an Ultrasound System". This duplex system measures only the component of velocity parallel to the insonifying beam.
Using the visual orientation facility of the instrument, together with the beamsteering capability provided in the Doppler mode of operation, it is possible in certain situations to manually position the single acoustic beam available so that it coincides with the blood flow direction in at least one major artery, but in many cases the alignment cannot be achieved. A principal object of this invention is to measure the true blood velocity vector, independent of the orientation angle .theta., by suitably modifying the foregoing duplex instrument which has a single linear transmit-receive transducer array for both B-scan and Doppler modes.